Mashup novels are all the rage right now, mixing zombies with classic lit or scifi with westerns. Last week at Comic-Con a panel of writers including Scott Westerfeld and Naomi Novik talked about the best and worst of mashup lit.

Novik, whose Temeraire books inject dragons into the Napoleonic wars, said what she likes about mashups is that they "feel familiar, but bring you the pleasure of something new." Also, it makes for incredibly fun worldbuilding.

Blood and Ice author Robert Masello said he came to genre mashups because he "always wanted to write my version of charge of the light brigade." The problem, he said, is that it's often hard to pitch these stories:

I mix historical fact with contemporary fiction. It's hard to explain it to a publisher, though. [I'll say], "People frozen in Crimean War are coming back to life and there's a vampire involved," and the publisher kind of looks at you – it's hard to explain these books, because it's all in the doing of it.

Justin Cronin, author of current bestseller The Passage says he "jumped the rails from mainstream literary fiction, [which is full of] novels where people have lots of earnest conversations over kitchen tables." With his new novel, which is about post-apocalyptic vampires, he says his goal was to mix the genres he loved as a kid, "post-apocalyptic novels of the Cold War, Westerns, thrillers and horror novels." He says ultimately genre fiction is "interesting." By which he meant, implicitly, that litfic isn't.

But Kraken and The City & The City author China Mieville said mashups don't always work. "I hate peanut butter cups," he said, making a face. "There's this problem of bad aesthetic arithmetic. Awesome plus awesome isn't always awesome. Occasionally it can lead to abomination." He added that genres are really marketing categories, so mashing them up just leads to more marketing. Still, he loves to play with multiple genres. "Genres have a set of protocols which they follow, even if they start as marketing, and those constraints are actually enabling," he said. Constraints foster creativity. And when you put two or more sets of constraints together, the creativity can be explosive.

Uglies and Leviathan author Scott Westerfeld said that young adult fiction isn't constrained by traditional categories like "horror" or "scif." Instead, "there are two genres for YA: boy and girl. Those do exist as literary paradigm. Uglies is about plastic surgery and 80 percent of the readers are girls. Leviathan appeals to boys – and my writing a boy book has caused a lot of resistance. Victorian biotechnology isn't as difficult for people to understand as the gender twisting that I'm doing."

An audience member asked whether there are any mashups that the authors wouldn't want to see. Most said there were none that couldn't be done, and Mieville joked about different kinds of weird mashups: "A dinosaur love story, or steampunk cookery." But Westerfeld finally hit upon the one kind of mashup that could never work: A driver's ed manual with an unreliable narrator.